This invention relates to electrical induction logging systems for determining the nature and characteristics of the various sub-surface formations penetrated by a borehole drilled into the earth. More particularly, this invention relates to a digital induction logging system for obtaining digital samples of signals characteristic of the resistivity of the formations.
It is important to the oil and gas industry to know the nature and characteristics of the various sub-surface formations penetrated by a borehole since the mere drilling of a borehole usually does not provide sufficient information concerning the existence, depth location, quantity, etc., of oil and gas trapped in the formations. Various electrical techniques have been employed in the past to determine this information about the formations. One such technique commonly used is induction logging. Induction logging measures the resistivity (or its inverse, conductivity) of the formation by first inducing eddy currents to flow in the formations in response to an AC transmitter signal, and then measuring a phase component signal in a receiver signal generated by the presence of the eddy currents. Variations in the magnitude of the eddy currents in response to variations in formation conductivity are reflected as variations in the receiver signal. Thus, in general, the magnitude of a phase component of the receiver signal, that component in-phase with the transmitter signal, is indicative of the conductivity of the formation.
In theory, the electrical resistivity of the formation should be relatively high when the formation contains a high percentage of hydrocarbons because hydrocarbons are relatively poor conductors of electricity. Where hydrocarbons are not present in the formations and the formations contain salt water, the electrical resistivity of the formation should be relatively low. Formation water, which typically is salty, is a relatively good conductor of electricity. Induction resistivity logging tools thus obtain information about the formations which can be interpreted to indicate the presence or absence of these hydrocarbons.
U.S. Pat. Nos. 3,340,464, 3,147,429, 3,179,879 and 3,056,917 are illustrative of typical prior-art well logging tools which utilize the basic principles of induction logging. In each of the tools disclosed in these patents, a signal generator operates to produce an AC transmitter signal which is applied to a transmitter coil. The current in the transmitter coil induces a magnetic field in the formations. This magnetic field, in turn, causes eddy currents to flow in the formations. Because of the presence of these formation currents, a magnetic field is coupled into a receiver coil R thereby generating a receiver signal. (Logging tools having "a receiver coil" and "a transmitter coil" each comprised of several coils arranged in a predetermined fashion to obtain a desired response are commonly used.) The receiver signal is then amplified and applied to one or more phase sensitive detectors (PSDs). Each PSD detects a phase component signal having the same phase as a phase reference signal which is also applied to the detector. The phase reference signal has a predetermined phase relationship to the current in the transmitter coil(s). The output of the PSD(s) may be further processed downhole, or may be sent uphole to surface equipment for processing or display to an operating engineer.
Heretofore, prior-art induction resistivity logging tools have been primarily analog in design, with some digital circuits used to perform some functions, e.g., see the digital flip-flops of U.S. Pat. No. 3,340,464. Because of the analog nature of prior-art designs and for other reasons, these prior-art tools have limitations which prevent them from meeting a growing need for more precise, accurate and error free measurements of phase component signals in the receiver signal.
A quantitative determination of the conductivity of the formations is based in large part on the value obtained for the phase component signal that is inphase with the transmitter current in the transmitter coil. This component signal is referred to as the real or "R" phase component. Measurement of a phase component signal which has a phase orthogonal to (or in other words, in quadrature to) the transmitter current is sometimes obtained. This component signal is referred to as the "X" phase component signal.
Measurement of both the R and X phase component signals of the receiver signal is known. U.S. Pat. Nos. 3,147,429 and 3,179,879 both disclose induction logging tools which detect phase quadrature components (V.sub.r and V.sub.x') of the receiver signal from the receiver coil. The tools disclosed in these patents show the output from a receiver amplifier being applied to identical PSD circuits, one for detecting the R component signal and the other for detecting the X component signal. Appropriate phase shifting components are provided for generating the phase quadrature phase reference signals required by the PSDs in order to resolve the phase component signals.
The need for higher precision and accuracy in the resolution of these phase component signals is a natural consequence of the need to know more about formation characteristics that can be extracted from the signals representative of these characteristics. But, to obtain accurate measurements, the inaccuracies present in the measurements obtained by the prior-art tools must be eliminated. A principal source of inaccuracies in the measurement of the R and X component signals present in prior-art logging tools results from phase shifts in the signals of the tool. These phase shifts result in a departure from the in-phase/quadrature phase relationship between the transmitter signal, the receiver signal and the phase reference signals, all of which are used in resolving the received signal into the quadrature component signals R and X.
Two principle sources of phase shift errors are present in induction logging tools--static phase shift errors and dynamic (temperature dependent) phase shift errors. Static phase shift errors are those phase shifts which occur when the tool is operating at a steady state temperature condition. These phase shift errors are introduced into the detected phase component signal by certain electrical circuits in the tool, i.e., the transmitter coil system, the receiver coil system, the amplifier used to condition the receiver signal and the PSD itself. The dynamic phase shift errors occur as a result of such influences as temperature drift in these same circuits, all of which are involved in the generation of the formation currents and in the detection of the phase components in the receiver signal. Unpredictable phase shifts may also be introduced by component variations that are an unavoidable consequence of the manufacturing process. High precision resolution of the component signals requires that these phase shift errors be automatically and periodically eliminated from the measurements during the logging operation. This is especially true since the temperature environment in which the induction tool is operated will vary over a wide range with the depth in the borehole.
The dynamic compensation for phase shift errors due to temperature drift in the circuits of an induction logging tool has been attempted in the prior art. U.S. Pat. No. 3,340,464 discloses a circuit for automatically adjusting for varying phase shifts due to temperature drift in the tool's circuits by deriving a test signal from the current in the transmitter coil; substituting this test signal for the normal receiver coil output signal; generating a quadrature reference signal to the PSD to detect a phase component (X) in the receiver signal; and, phase shifting the reference signal as a function of the magnitude of the detected phase component signal in a direction to minimize that signal. This disclosed phase error compensation circuit and method does not attempt to segregate the relatively fixed or constant phase errors of the tool from the temperature dependent phase errors which vary with time during logging and resulting from component drift in the circuits. Rather, the tool of U.S. Pat. No. 3,340,464 attempts to compensate for any and all phase shifts regardless of their source which have occurred since the last phase compensation.
As a result, the phase compensation circuit of U.S. Pat. No. 3,340,464 must compensate for the phase angle error over a greater range of angles than would be required if the static and temperature dependent phase shift errors were separately compensated. A large range in phase angle compensation results in less sensitivity to small phase shift errors. This loss of sensitivity allows uncompensated phase shift errors to appear in the detected phase component signal. These errors prohibit the high precision and accuracy in the measurements.
Those prior-art tools, such as those disclosed in U.S. Pat. Nos. 3,147,429 and 3,179,879, which measure both R and X require two PSDs, one for measuring R and one for measuring X. This dual arrangement of detecting circuits in an induction tool implies that the static and temperature dependent phase shift errors for each of the two PSD's will not be the same, i.e., the circuits will not respond identically to a given temperature change even if they could be made to have the same phase shift at a given temperature. Because of this difference, different phase shift errors will be present in the R and X measurements. Even with phase shift compensation techniques, such as that disclosed in U.S. Pat. No. 3,340,464, applied to the PSD circuits, one compensation circuit could not compensate for both detectors. Two compensation circuits would be required, one for each PSD. This, of course, would increase significantly the circuit complexity of the induction tool and a reduction in its overall reliability.
It is a characteristic of induction tools that at low conductivities, the amount of direct mutual coupling ("X" sonde error) between the transmitter coil and the receiver coil, even in a tool which employs a system of receiver coils which minimize this mutual coupling, is not zero. In fact, a ratio of 10:1 of the signal response due to direct mutual coupling to the R component in the receiver signal is not uncommon. When encountering low conductivities, in order to resolve the R component to .+-.1% accuracy, a phase accuracy of 1 milliradian is required. For the case of high conductivities, the R component will exceed X by a factor which can be substantial, i.e., "R"=10.times."X". For this case, to resolve X to .+-.1% would likewise require a high degree of phase accuracy.
To obtain accurate phase component signal measurements that are essentially free of the static and temperature dependent phase shift errors, a highly phase stable, low distortion transmitter signal must be generated. A highly phase stable transmitter signal is required to insure phase accuracy between the signals of the tool in the generation of the transmitter signal and in the detection of the phase component signals in the receiver signal. The requirement for low distortion in the transmitter signal results from the frequency response of the earth's formations.
A known phenomenon in induction logging is the difference in the formation response as a function of frequency and formation conductivity. In general, the response signal received by an induction tool at low conductivities increases as the square of the frequency for a constant transmitter current. Because of the greater formation response at higher frequencies than at lower frequencies over most of the conductivities encountered, it becomes apparent that a low distortion transmitter signal is required. The more distorted the transmitter signal is, the larger in amplitudes are the harmonics of the fundamental frequency. Such harmonics propogate through the formation from transmitter to receiver with an attenuation and phase shift not related to those of the fundamental frequency. They can thus introduce false signals into the receiver that may cause a misleading result to be obtained from the induction tool measurement. Thus, more noise will be present in the resulting receiver signal from these higher frequency harmonics.
This variation in formation response with frequency can be put to good use to extend the range of formation resistivity that may be accurately measured by an induction logging tool. At high formation conductivities and higher frequencies, a phenomenon known as "skin-effect" causes a loss of proportionality between the received signal and formation conductivity, introducing additional complexity in the interpretation of the signals.
Additionally, at the lower transmitter frequencies and at low conductivities, the response from the formation falls below the noise level of the induction logging system. In this case, meaningful measurements are impossible. Thus, when encountering low conductivities, a high frequency for the transmitter signal would provide the more accurate reading of the formation conductivity. But, because of the sloping away of the response curves for the higher frequencies at higher conductivities, it would be desirable to have a lower transmitter frequency at high conductivities to avoid ambiguity in the conductivity derived from those measurements. This may be achieved by selection of a single frequency appropriate for the conductivity range expected prior to logging, or by the generation of two or more frequencies simultaneously in the transmitter, with subsequent frequency separation in each receiver circuit and in each phase selective detection circuit, or by sequentially switching to different frequencies while logging.
Yet another problem present in prior-art logging tools has been the problem of determining from the measured tool output responses the true and correct characteristic of the formation. That is, determining the transfer function of the tool relating the tool input signal, representative of the formation characteristic, to the measured tool output response. It is from this transfer function that the true value of the formation characteristic is inferred based on the measured output responses.
Because of variations in circuit parameters as a result of temperature changes, (e.g., changes in the amplifier gains) the calibrated transfer function of the tool at one operating position may not be the same as at another. A determination of the transfer function is normally effected uphole by placing one or more signal sources near the receiver coil to simulate various formation conductivities. The responses to these test signals are recorded and used to derive a calibration transfer function for the tool. This function is thereafter used as the function relating input to output of the logging tool. Yet, for prior-art tools, the data obtained during a logging run is not corrected for the effects of temperature changes, during logging, to the transfer function.
A further characteristic of all induction logging tools is the very wide dynamic range present in the detected phase component signals over which useful information is contained. A dynamic range of 10,000:1 (&gt;80 db) is not uncommon. Superimposed on the useful information in a detected component signal is a certain amount of random noise which degrades the quality of any measurements made. In analog prior-art induction logging tools (as distinguished from a digital logging tool), this noise includes noise generated during the transmission of the detected analog phase component signals to the surface through a wireline logging cable. Analog transmission of the phase component signals uphole is subjected to the problem of signal degradation by the introduction of error potentials and noise or cross talk in the electrical leads of the logging cable.
Prior-art logging tools have attempted to handle the large dynamic range in the detected component signals in different ways. U.S. Pat. No. 3,056,917 discloses one such technique in which the dynamic range is divided into two parts--a first range in which the transmitter current is adjusted to obtain a constant receiver signal voltage and a second range in which the transmitter current is held constant. A signal is then recorded which is representative of the transmitter current when the receiver signal is constant, and which is representative of the receiver signal when the transmitter current is held constant. The resulting recorded signal represents the conductivity of the formation in the first range and the resistivity of the formation in the second. Yet other prior-art techniques for handling this large dynamic range in the detected phase component signals are also discussed in U.S. Pat. No. 3,056,917.
Most prior-art tools have used standard techniques to try to eliminate or minimize the amount of noise introduced into the analog signals transmitted over the logging cable. The use of twisted wire pairs, shielded leads, low noise slip rings, etc. are but a few. Where an induction tool requires precise, accurate measurements of the detected signals, regardless of their magnitudes, these prior-art techniques are no longer adequate.
Because of the limitations present in the prior-art logging tools and the need for more precise and accurate measurements of the phase quadrature components of the receiver signal, it would be advantageous to provide an induction logging tool to measure and convert to digital form downhole the wide dynamic range in the detected phase component signals, and to measure them with the same resolution and accuracy at all levels of signals. These digital signals are subsequently transmitted to the surface substantially uncorrupted by noise as previously discussed. It would also be advantageous to dynamically compensate for both the static and temperature dependent phase shift errors in the circuits of the tool involved in the generation of the formation currents and in the detection of the phase components of the receiver signal.
It would also be advantageous to provide an induction tool which digitally generates downhole both a highly phase stable, low distortion transmitter signal and a highly stable phase reference signal in order that a single phase sensitive detector may sequentially detect both the R and the X phase quadrature component signals while compensating for the phase shift errors. It would also be advantageous to provide a digital induction logging tool in which the frequency of the digitally generated transmitter signal is selectable from among a . plurality of transmitter frequencies. It would also be advantageous to provide an induction tool which automatically selects, during a logging run, the transmitter frequency or frequencies that will produce the optimum formation response signals for the conductivities actually encountered by the tool. It would also be advantageous to provide an induction logging system which automatically produces, during a logging run, test calibration measurements which are used to derive a linearization correction function to correct for temperature dependent variations in the transfer function of the tool at any time during the logging run.